Dredging operations in navigable waterways are based upon the rate of sediment deposition. The study of the rate of sediment deposition can be greatly enhanced by collecting unbiased water-sediment samples from rivers and streams.
The life expectancy of a water reservoir is determined by the rate at which it is filled with sediment from the rivers and streams that feed it. In order to determine the rate of sediment deposition in a water reservoir, it is necessary to have a means to take accurate and reliable samples from the flowing waters that feed the reservoir.
The U.S. Government is currently involved in comprehensive studies of the quality of the rivers in the United States. For the past 30 years the U.S. Geological Survey has conducted the National Stream Quality Accounting Network (NASQAN). The NASQAN provides information for tracking water quality conditions in major U.S. rivers. Another federal program is the National Water Quality Assessment (NAWQA) program, conducted by the U.S. Geological Survey. The NAWQA program is designed to assess the status and trends in the quality of the nation's ground- and surface-water resources and to develop an understanding of the major factors that affect water quality conditions.
In the early days of fluvial-sediment investigations, each investigator or agency responsible for investigation developed methods and equipment individually as needed. It became apparent that reliable and consistent data could not be obtained unless equipment, collection methods, and analytical methods were standardized. To overcome this difficulty, representatives of several Federal agencies met in 1939 to form an Interdepartmental Committee to standardize sediment data-collection equipment and methods and analytical equipment. The committee was reorganized in 1956 to its present structure as the Federal Interagency Sedimentation Project (FISP) (Edwards and Glysson, 1988).
Since its initiation in 1939, FISP has published more than 50 reports dealing with nearly all aspects of measurement and analysis of fluvial sediment movement. FISP has also set up several criteria for the design and construction of suspended-sediment samplers:
1. Water must enter the sampler nozzle isokinetically. In isokinetic sampling, water approaching the sampler nozzle of the sampler undergoes no change in speed or direction as it enters the nozzle.
2. The sampler nozzle must be permitted to reach a point as close to the streambed as is physically possible.
3. Disturbance of the flow pattern of the stream must be minimized, especially at the nozzle.
4. The sampler must be adaptable to support equipment already in use for streamflow measurement.
5. The device must be as simple and maintenance free as possible.
6. The sampler must accommodate a standard bottle size, i.e., 1-pint (473 ml) glass, 1-quart (946 ml) glass, 1-liter plastic, or 3-liter plastic.
Isokinetic samplers are divided into two categories according to how they sample: those that are depth-integrating and those that are point-integrating. A depth-integrating sampler is designed to accumulate a water-sediment sample from a stream vertical at such a rate that the velocity in the intake nozzle is essentially equal to the incident stream velocity while transiting the vertical at a uniform rate (FISP, 1952). The resulting water-sediment sample collected is proportional to the instantaneous stream velocity at the locus of the intake nozzle and therefore will be representative of the sediment load in the vertical. A simple depth-integrating sampler fills while it is being lowered from the water surface to the streambed and while being raised to the surface again.
At any instant during the operation of a depth-integrating sampler, the air mass in a rigid container is a function of the hydrostatic head and the volume of water-sediment collected. As the sampler is lowered into a stream, sufficient water must enter the container to compress instantaneously the inside air so that its pressure balances the external hydrostatic head according to Boyle's law. For the water-sediment inflow in the nozzle to be equal to the stream velocity, the rate of air volume contraction due to increasing hydrostatic pressure must not exceed the normal volume rate of liquid inflow. As a result, the sampler must be lowered and raised in the water column at a rate such that these two factors are balanced to avoid the water-sediment mixture being forced into the sampler at a velocity greater than the ambient stream velocity. This vertical rate is know as the transit rate. Studies have shown that its value must not exceed 0.4 times the stream velocity due to the apparent approach angle of the nozzle facing into the stream as the sampler makes its vertical descent and ascent (FISP, 1952).
Other studies of the filling characteristics of the rigid-bottle container have shown that the maximum distance the sampler can travel through the water column and still sample is isokinetically is 34 ft at sea level (FISP, 1952). Since the depth-integrating sampler collects water from the instant it enters the stream, the maximum theoretical stream depth that can be sampled is half of this distance, or approximately 17 feet. General field practice limits the use of depth-integrating samplers to 15 feet (Edwards and Glysson, 1988).
FISP has designed depth-integrating rigid-bottle samplers that have been used for many years. These are designated as the U.S. DH-48, a 1-pint hand held sampler; the U.S. DH-59, a 1-quart hand line sampler; and the U.S. D-77, a 3-liter cable-suspended sampler.
Rigid-bottle samplers are limited to a depth of 15 feet. Additionally, rigid bottle samplers are limited in transit rate due to the air compressibility problems associated with rigid-bottle samplers.
Suspended-sediment samplers using a collapsible bag have been investigated as an improvement over the U.S. series of depth-integrating samplers. Several investigators have researched collapsible bag samplers. Two early models were developed by Gluschkoff (FISP, 1940) and by the Rhine Works Authority (FISP, 1940). The Gluschoff sampler, developed in Russia, consists of several balloon-shaped bags, each fitted with a nozzle. The nozzles were mounted on a vertical staff and oriented horizontally in the same direction. When sampling, the staff was inserted into the stream with the nozzles facing downstream and with the bags devoid of air. The staff was then twisted so that the nozzles faced upstream. The bags simultaneously collected point-integrated samples at preselected depths. The staff was again twisted so that the nozzles faced downstream, pinching off any further inflow. The staff was carefully lifted out of the water and the samples removed. The major problem with the arrangement was that the bags were unprotected and had to be handled very carefully.
The Rhine Works Authority sampler consisted of a latex balloon, a nozzle, and a metal frame with a tail fin. When sampling, a pinch clamp located at the neck of the balloon was operated by an auxiliary line to allow flow into the balloon. The sampler was not streamlined, and the requirement for an auxiliary line limited the use of the sampler.
Stevens and others (Stevens et al, 1980) fabricated 1-gallon and 2-gallon samplers using plastic bags. The samplers consisted of a wide-mouth, perforated, rigid container enclosed in a cage-like metal frame attached above a sounding weight. The head of the frame supported a plastic intake nozzle and swung open to permit the plastic container to be removed. When the head was closed, the end of the nozzle extended slightly into the mouth of the container and the container sealed against a gasket. An adjustable rubber stop at the rear of the sampler held the container in place. The perforations in the container were 0.75 in diameter holes arranged in three partial rings of six holes each on the underside of the container at different lengths. In addition, there was a large opening in the side of the container just below its neck. During sampling, this opening was covered with a loose fitting plastic sleeve. For sampling, a collapsed, pre-shaped, flexible, plastic bag was placed inside the rigid container. The neck of the flexible bag was stretched over the neck of the rigid container, and the whole unit placed into the sampler. The sampler was of limited use at stream velocities above 3 ft/sec, was cumbersome to operate, and had an unsampled zone of approximately 18 inches.
Szalona (1982) conducted another investigation into the use of a bag sampler. His approach was to modify the U.S. D-77 sampler. The sampler was equipped with a 3-liter plastic bottle, nozzle cap and nozzle, and used a commercially available food storage bag. Holes were drilled in various locations of the bottle to enable quick flooding of the bottle. Various combinations of vents and deflectors were added to the U.S. D-77 sampler to facilitate isokinetic sampling. The sampler had limited use at stream velocities above 3 ft/sec. However, its sampling capacity is limited to approximately 2.5 liters. Some difficulty was also encountered trying to remove the bag filled with sample through the small opening of the bottle mouth. Additional testing by FISP and experience by field personnel has shown that if the collapsible bag is not placed correctly inside the container, the sampler will not sample at all.
In a recent study of contaminants in the Mississippi River, Robert Meade used an 8-liter frame-type bag sampler similar to that described by Stevens (Meade). This sampler consisted of a perforated 8-liter plastic container with a U.S. D-77 sampler cap and nozzle secured inside a metal frame. The metal frame was suspended above a sounding weight. A collapsed 8-liter perfloroalkoxy (PFA) bag was placed inside the plastic container. Analysis of the sampling data showed that the sampler collected water-sediment at a rate that approached isokinetic (ideal plus or minus 15 pct) in only about half of the samples.
Jobson, in U.S. Pat. No. 5,693,894, discloses a fluid controlled isokinetic fluid sampler comprising an inflatable bag provided within a hollow fluid-tight housing which is filled with water or other appropriate liquid. An inlet tube permits a sample of a flowing fluid to be introduced into the inflatable bag from the exterior of the housing to inflate the bag within the housing. A pump is provided to pump the water from within the housing through an outlet tube to the exterior of the housing at a flow rate proportional to the flow rate of the flowing fluid.
Kozak, in U.S. Pat. No. 4,888,999, discloses a tank bottom sampling device comprising an outer cylindrical body having a suspending handle and a central opening through which a piston assembly is positioned. As the device is lowered into a tank bottom, an extension at the bottom of the piston contacts the tank bottom and the piston moves upward to allow liquid to flow into the sampling device. This device is not designed to be used in flowing waters.
Inking, in U.S. Pat. No. 4,302,974, discloses a water sampling device comprising a pliable container or bag initially mounted in a deflated condition on a framework. The framework includes sealing means for securing the pliable container with at least a portion of the container in a partially rolled condition to seal an opening in the container. The framework further includes opening means for unrolling the container to expose the opening to the container at the desired water sampling depth. A pair of wing members mounted to the framework spread apart the sides of the container to draw water through the unsealed opening. After the container has been substantially filled with water, resealing means of the framework rolls the container back up to seal the container at the sampling depth.
Burney, in U.S. Pat. No. 4,606,233, discloses a bag sampler comprising a rigid plastic frame and a flexible polyethylene bag. The frame holds the bag open during filling and allows it to close without using rigid moving parts. The frame comprises a bag-retaining flange bonded to a cylindrical central core attached to four heavy struts which support a large hoop frame at the front end thereof. The central core is a sealed hollow chamber, which is penetrated by an internal sampling tube with polypropylene compression tube fittings at both ends. The sampling tube passes through the central core, extends a few centimeters behind the retaining flange, and is held in place by both the front and rear compression fittings. A plugged port in the forward end of the central core allows the change to be ballasted with water to achieve neutral buoyancy. A plurality of holes provide attachment points for a towing harness.
Unfortunately, many of the bag samplers described above were not consistent in sampling. Sometimes they would obtain a sample and sometimes they would not, for no apparent reason. Additionally, they would only collect a sample volume approximately 80 percent of their rated volume. They had a large unsampled zone (the distance from the streambed to the intake nozzle). They would not sample effectively at stream velocities below approximately 3 ft/sec.